The invention relates to an object, which is transparent for visible light and reflects infrared radiation, comprising a transparent plastic substrate coated with an infrared light reflecting coating, wherein the coating comprises IR reflecting particles which comprise a titanium dioxide layer applied on a flake like carrier.
The invention also relates to a process for the manufacture of such object.
EP-B-548822 describes plastic objects comprising a rigid amorphous base material made of a light-transmissive synthetic material and a coating layer with IR reflecting particles. EP-B-548822 deals with objects that have a transmission in the visible range (T) of 45 to 75% and a total energy transmissivity (g) of 30 to 60% in respect of radiation energy and a solar factor according to DIN 67507, expressed by the ratio T/g of more than 1.15. To obtain the above described optical properties it is according to EP-B-548 822 necessary to use in the coating layer red IR reflecting particles which comprise one titanium dioxide layer of a thickness of 60 to 120 nanometer, applied onto a flake like carrier.
The objects of EP-B-548822 have the disadvantage that they show a pronounced color flip flop, which is undesirable for many applications. The expression xe2x80x9ccolor flip flopxe2x80x9d is used to describe the following: upon looking under different angles at the coated surface of the products of EP-B-548822 the color changes. This is called color flip flop. Looking at an angle of 90xc2x0 the color is in the case of the products of EP-B-5488222 red. Looking at the surface under a very small angle of a few degrees the color is light green. At intermediate angles occurs a shift in color.
The present invention deals with objects of the kind described in EP-B-548822 which do not show or hardly show the above described color flip flop.
The present invention further deals with a very simple process for the manufacture of the claimed objects.
The objects of the invention are provided with an IR reflecting coating comprising IR reflecting particles comprising at least two titanium dioxide layers applied on a flake like carrier.
By proper selection of all relevant parameters it is possible to obtain objects with the same good optical properties as the products of EP-B-548822, but which do not or hardly show the color flip flop of the products of EP-B-548822.
The following parameters are in this respect of relevance to determine the optical properties of the object of the invention.
1) thickness of the coating and concentration of IR reflecting particles in the coating;
2) thickness and number of titanium dioxide layers on the flake like carrier;
3) particle dimensions of the flake like carrier;
4) orientation of the IR reflecting particles;
5) characteristics of the binder in the coating;
6) characteristics of the substrate.
The infrared reflecting particles used in the coating of the objects of the invention are known as such. They have been described for example in DE-A-19618569. The infrared reflecting particles used in the coating of the objects of the invention differ with respect to the particles used in the coating of EP-B-548822 in that they have more than one layer of titanium dioxide, which has been applied upon a flake like carrier. The titanium dioxide layers are separated from one another by layers with a refractive index, which is different from the refractive index of the titanium dioxide. Suitable layers can be made of SiO2, AL2O3 and the like. The titanium dioxide layers and the separating layers are applied upon a flake like carrier. The flake like carrier can be made for example of aluminium flakes or mica flakes.
The dimensions of the flakes can be varied within broad ranges. The thickness will usually be selected between 100 and 3000 nanometer, more preferably between about 200 and 2000 nanometer. The diameter will usually vary between 5-200 micrometer, more preferably between 5-100 micrometer. Smaller diameters usually result in less transmission for visible light.
The thickness of the titanium dioxide layers on the carrier can also vary. Suitable ranges for each titanium layer vary between 50 and 150 nanometer, more preferably between 80 and 120 nanometer.
The number of titanium dioxide layers should be at least two. It can be for example two, three, four, five or more. The number is preferably two.
Suitable infrared reflecting particles are commercially available for example form Merck KgaA under the designation Iriodin AC 870.
The coating composition comprises besides the infrared reflecting particles usually a binder. Suitable binders are polymeric materials. They can be made for example of polymerisates of acrylic or methacrylic esters, polycarbonates, polyurethanes, polyesters, polystyrene and polyvinylchloride. The binder is most preferably transparent for visible light. The choice of the binder will depend upon the chemical nature of the substrate and the process for applying the coating on the substrate. For substrates made out of polycarbonate it is preferred, in particular when the coating is applied by coextrusion, to use a polycarbonate binder.
The concentration of the infrared reflecting particles in the coating can vary within broad ranges. To achieve optimal optical properties a higher concentration of infrared reflecting particles can be combined with a relatively thin layer of the coating or a lower concentration of infrared reflecting particles can be combined with a relatively thick layer of the coating.
The concentration of the infrared reflecting particles in the coating will usually vary between 10-50, more preferably 20-40% by weight (on the basis of the dry weight of the coating).
The thickness of the coating on the substrate can vary between 1 to 100, more preferably 5-50 micrometer.
The coating can cover the complete surface or only a part of the surface of the object. In a preferred embodiment of the invention the objects of the present invention are flat or curved sheet or film. In said case one or both surfaces can be coated either completely or partly.
The substrate can be made out of any transparent plastic material, which is transparent for visible light. The substrate preferably has a transmission of at least 50% for visible light. The transmission depends on the material of the substrate and on the thickness thereof. Suitable materials for the substrate are the same as those mentioned above for the binder.
The substrate is preferably in the form of a flat or curved sheet or film. The sheet can be provided with many hollow channels.